This invention relates to novel fine amorphous powder and a process for preparing fine powdery mixture of silicon nitride and silicon carbide.
Silicon nitride or silicon carbide ceramics are distinguished in high temperature characteristics such as high temperature strength, high thermal shock resistance, etc., as compared with oxide-based ceramics including alumina as a typical oxide ceramic, and extensive research has been so far made for their processes and applications. Their uses as high temperature materials such as heat-resistant structural materials for gas turbines, diesel engines, heat exchangers, etc. working at a high temperature have been promising.
However, silicon nitride and silicon carbide ceramics have the following drawbacks in spite of the distinguished high temperature characteristics. For example, silicon nitride ceramics having a distinguished thermal shock resistance have poor mechanical strength and oxidation resistance at a high temperature, whereas silicon carbide ceramics having a distinguished oxidation resistance have a poor thermal shock resistance.
To improve these drawbacks, processes for preparing composite ceramics containing silicon nitride and silicon carbide have been studied, and for example, the following processes are known:
(1) A process comprising mechanically mixing silicon nitride powder with silicon carbide powder, and sintering the mixture by hot pressing.
(2) A process comprising molding a mixture of silicon carbide and silicon in advance, and then subjecting the molding to nitriding reaction, thereby forming silicon nitride portions, or molding a mixture of silicon nitride and carbon in advance and then subjecting the mixture to silicon permeation, thereby forming silicon carbide portions, both according to a sintering reaction.
(3) A process comprising adding silicon powder to organosilicon polymers as a starting material, molding the mixture, and then subjecting the molding to a nitriding reaction, thereby forming both silicon carbide portions and silicon nitride portions.
However, these attempts still have the following drawbacks: when the conventional starting material powder is used, there is a limit to thorough control of mixing degree as well as particle characteristics such as particle size, shapes, etc., and also to uniform mixing of the component particles; the starting material powder is liable to be contaminated with impurities owing to the mechanical pulverization and mixing, and a desired satisfactory sintered product may not be obtained; even the reaction sintering may make the sintered product porous, or complicate the steps or operations, or may have a limit to the homogeneity of the composition, and a desired satisfactory sintered product may not be obtained.